2 answers

Since you have no transition screen, there is no way for the workflow to associate any field updates with that transition, which is why it throws errors saying you've performed "an illegal operation." If you add a transition screen (even if it only has the Comment field visible), does that fix the issue?

To expand on what Tyler has said. To create a comment screen, you just need to create a screen, and put no fields on it. The comment is automatically added to the screen by JIRA. I know, it seems counterintutive but it does work, trust me.

Now when you add the comment screen to your workflow transition, it will display only a comment field. Enter your comment there and your validation should now work.

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Another way you could do this, assuming that you just want to have at least 1 comment on the issue, regardless of when it was added, is create a custom field of type "number of comments". Then have a validator to check that this field has a value greater then 0.

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